In a control device of an inverter which drives a magnet-type synchronous motor, to realize the downsizing, the reduction of weight, the reduction of cost and the enhancement of reliability of the control device, there has been proposed a rotary sensorless control method which does not use a rotary sensor such as a resolver or an encoder. In the rotary sensorless control, it is desirable that a rotary phase angle and a rotational speed can be estimated within a wide speed range from stopping of the inverter to a maximum high speed.
For example, in applying a rotary sensorless control to a synchronous electric motor having a magnet in a rotor, positional information of a magnetic pole (at least one of an N pole and an S pole) of the magnet is indispensable.
However, in a method which uses a harmonic current, there has been a case where it is difficult to detect the harmonic current during a high speed rotation so that accuracy of determination of the position of a magnetic pole of the magnet is lowered. Further, there has been a case where applying of a harmonic voltage generates noise.
Further, even with respect to an electric motor of a method which uses a magnet induced voltage, in the case of an electric motor having a small magnet magnetic flux, for example, an electric motor which positively makes use of a reluctance torque, even when the electric motor is rotated at a high speed, a non-load induced voltage is scarcely generated and hence, it is difficult to determine a magnetic pole position of a magnet by making use of a voltage signal.